Field of the Invention
The invention relates to a method for processing a polyester resin that includes melting and subsequently solidifying the polyester resin to form a shaped article without decreasing the intrinsic viscosity of the polyester resin by more than 0.025 dL/g. The invention further relates to molded articles prepared by the method and to the polyester resins capable of undergoing processing without a decrease in intrinsic viscosity of more than 0.025 dL/g.
Description of the Related Art
Polyester resins including resins such as poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), poly(ethylene naphthalate) (PEN), poly(trimethylene terephthalate) (PTT), and poly(trimethylene naphthalate) (PTN), are conventionally used as resins in the manufacture of containers such as beverage bottles. Properties such as flexibility, good impact resistance, and transparency, together with good melt processability, permit polyester resins to be widely used for this application. The term resin as it is used herein includes all of the aforementioned materials.
The starting feedstocks for polyester resins are petroleum derivatives such as ethylene, which is obtained from petroleum or natural gas, and para-xylene, which is typically obtained from petroleum.
Polyester resins are generally made by a combined esterification/polycondensation reaction between monomer units of a diol (e.g., ethylene glycol (EG)) and a dicarboxylic acid (e.g., terephthalic acid (TPA)). The terms carboxylic acid and/or dicarboxylic acid, as used herein, include ester derivatives of the carboxylic acid and dicarboxylic acids. Esters of carboxylic acids and dicarboxylic acids may contain one or more C1-C6 alkyl groups (e.g., methyl, ethyl, propyl, iso-propyl, butyl, iso-butyl, tert-butyl, pentyl, hexyl and mixtures thereof) in the ester unit, for example, dimethyl terephthalate (DMT).
In conventional esterification/polycondensation processes, polyester may be formed, for example, by first producing a prepolymer of low molecular weight and low intrinsic viscosity (IV) (e.g., a mixture of oligomers), for example, by reacting a diol and a dicarboxylic acid in a melt phase reaction. The formation of the oligomers may be carried out by reacting a slurry of diol and dicarboxylic acid monomer units in an esterification reactor. EG may be lost to evaporation during the esterification reaction which may be carried out at high temperatures. Therefore the slurry of diol and dicarboxylic acid may contain an excess of EG, for example the diol and dicarboxylic acid may be present in a molar ratio of from about 1.2 to about 2.5 based on the total glycol to total di-acid. Further pre-polycondensation and polycondensation of the oligomers can be carried out to provide a resin mixture having an IV of from 0.50 to 0.65. Such resin mixtures are suitable in various applications such as fibers/filaments, fiber chips, or bottle-resin precursors. Amorphous clear base chips having an IV of from 0.50 to 0.65 may be subjected to solid-state polymerization (SSP) to increase the molecular weight (e.g., to an IV of from 0.74 to 0.76 for water bottle applications, 0.83 to 0.85 for CSD/Beer bottles, etc.). The solid-state polymerization (SSP) process unit can result in the resin undergoing crystallization which forms opaque pellets.
A continuous polyester melt-phase polycondensation process usually consists of three reaction steps: (i) esterification to form low molecular weight oligomers, (ii) pre-polymerization of the oligomers to form a pre-polymer, and (iii) polycondensation to form a polymer with an intermediate molecular weight or intrinsic viscosity (e.g., a target intrinsic viscosity of from 0.50 to 0.65).
The three reaction steps (i), (ii), and (iii) above, can be carried out to achieve the target intrinsic viscosity in from 2 to 6 reactors using existing melt-phase process technology. In general, esterification is conducted in one or two vessels to form a mixture of low molecular weight oligomers with a low degree of polymerization (e.g., about up to 7 monomer unit pairs reacted). The oligomers are then pumped to one or two pre-polymerization vessels where higher temperatures and lower pressures aid in removing water and EG. The degree of polymerization then increases to a level of 15 to 20 repeating units. The temperatures are further increased and pressures are further reduced in the final one or two vessels to form a polymer ready to be cut into pellets for example, or to be spun directly into fibers or filaments.
Esterification and pre-polymerization vessels may be agitated. Polycondensation vessels (e.g., finishers, wiped-film reactors etc.) may have agitators designed to generate very thin films. Temperatures and hold-up times are optimized for each set of vessels to minimize the degradation and other side reactions. Some by-products that may be generated by the polyester melt phase reaction include diethylene glycol (DEG), acetaldehyde, water, cyclic oligomers, carboxyl end groups, vinyl end groups, and anhydride end groups.
Both time and temperature are two variables that are preferably controlled during an esterification/polycondensation reaction. With higher reaction temperatures, the total reaction time is significantly reduced and less residence time and/or fewer reactors are needed.
Alternatively to such a continuous production method, polyesters may be prepared using a batch method. In a batch method the diol and dicarboxylic acid units are mixed together in a single reactor. In some cases more than one reactor (e.g., reaction vessel) may be used if necessary. The diol/dicarboxylic acid mixture is heated to cause the monomer units to undergo a condensation reaction. The by-products of the condensation reaction may include water or an alcohol. By conducting the reaction under reduced pressure or by subjecting the reaction mixture to reduced pressure during the final stages of the reaction, volatile by-products of the reaction can be removed thus driving the reaction to completion.
Certain physical and chemical properties of polymeric materials are negatively affected by long exposure to elevated temperature, especially if the exposure is in an oxygen-containing atmosphere or at temperatures above, for example, 250° C. Conventional methods for preparing polyester resins such as PET may suffer from disadvantages associated with the need to carry out a solid state polymerization (SSP) which subjects the resin to a long heat history and/or may require high capital expenditure.
The production of a polyester resin such as PET may be carried out directly from a melt phase of the monomer units without any final solid-state polymerization. For example, a batch process may be carried out at a sufficient temperature, for a sufficient time and at a sufficient pressure to drive the polycondensation reaction to completion thus avoiding the need for any subsequent finishing (e.g., final reaction).
Solid-state polycondensation is an important step in some conventional processes used to manufacture high molecular weight polyester resins for bottle, food-tray, and tire-cord applications. The clear amorphous pellets (0.50 to 0.65 IV) produced by conventional melt polycondensation reaction processes may be further polymerized in the solid state at a temperature substantially higher than the resin's glass transition temperature but below the resin's crystalline melting point. The solid state polymerization is carried out in a stream of an inert gas (usually nitrogen under continuous operation) or under a vacuum (usually in a batch rotary vacuum dryer). At an appropriate SSP temperature, the functional end groups of the polymer (e.g., PET) chains are sufficiently mobile and react with one another to further increase the molecular weight.
A conventional process for producing polyester resins for container applications including melt-phase polycondensation and solid state polymerization is shown schematically in FIG. 1 wherein the monomer components of a polyester resin such as PET are mixed in a melt-phase esterification/polycondensation reactor. The reaction is carried out to provide a molten resin having an intrinsic viscosity (IV) of from 0.5 to 0.65. The molten product obtained by the melt-phase esterification/polycondensation is then subjected to a polymer filtration. Optionally a co-barrier resin may be added to the filtered, molten polymer by extruding the co-barrier resin and adding the extrudate to the filtered, molten resin obtained from the melt-phase esterification/polycondensation. The mixed streams, or the polyester stream obtained from polymer filtration may then be pumped into a mixer. A static mixer may be used to ensure that the polyester resin and any co-barrier resin are sufficiently mixed.
The melt-phase esterification/polycondensation is typically carried out in a plurality of reactors. Therefore, the monomers may be added to a first esterification reactor to form a low IV material. As the oligomers pass through the remaining reactors, the IV is subsequently raised as the polycondensation reaction proceeds sequentially through a series of reactors. The material in molten form that is pumped from the static mixer is subjected to solidification and pelletizing. The molten material may be solidified by passage of strands or filaments of the material formed by pumping the material through, for example, a die with a series of orifices. As the molten polyester resin is passed through an orifice, a continuous strand is formed. By passing the strands through water, the strands are immediately cooled to form a solid. Subsequent cutting of the strands provides pellets or chips which, in a conventional process, are then transferred to a solid-state polymerization stage (i.e., SSP).
In conventional processes for preparing polyester resins and even in some processes which avoid the use of a solid-state polymerization after polymerization is complete, the molten polymerized resin may be pumped through a die to form multiple strands. The molten resin exiting from the die is quickly quenched in water to harden the resin. As a result of the quick cooling (e.g., water quench) the molten polyester does not have time to crystallize and is solidified in an amorphous state. Solidified polyester strands, or pellets derived from cut strands, are clear, transparent and in an amorphous state.
The SSP may include several individual reactors and/or processing stations. For example, the SSP may include a pre-crystallization step wherein the chips and/or pellets are transformed from an amorphous phase into a crystalline phase. The use of a crystalline phase polyester resin is important in later steps of the SSP because the use of amorphous polyester chips may result in clumping of the pellets since an amorphous state polyester resin may not be sufficiently resistant to adherence between pellets and/or chips. The SSP process further includes a crystallizer (e.g., crystallization step), a pre-heater, a cooler, and an SSP reactor.
Some manufacturing processes do not include an SSP. Processing a polyester resin directly from a melt phase condensation to obtain pre-forms for blow molding applications is described in U.S. Pat. No. 5,968,429 (incorporated herein by reference in its entirety). The polymerization is carried out without an intermediate solidification of the melt phase and permits the continuous production of molded polyester articles (e.g., pre-forms), from a continuous melt phase reaction of the starting monomers.
After pre-crystallization, the chips and/or pellets may be subjected to a final crystallization. A final crystallization may include, for example, proper heating of the chips (pellets, pastilles, granules, round particles, etc.) at appropriate temperatures. Once the polyester resin is in a crystallized state, the pellets and/or chips are preheated and ready for transfer to the top of a counter-flow SSP reactor (parallel to the pre-heater) via a pneumatic system (e.g., Buhler technology). If a tilted crystallizer is stacked above the SSP reactor, the hot/crystallized chips then enter the SSP reactor by the rotating screw of the crystallizer (e.g., Sinco technology). The SSP reactor can be considered as a moving bed of chips that move under the influence of gravity. The chips have a slow down-flow velocity of from 30 to 60 mm/minute and the nitrogen has a high up-flow velocity of about 18 m/minute. A typical mass-flow ratio of nitrogen to PET is in the range of 0.4-0.6. In a gravity-flow reactor, the pellets and/or chips are subjected to elevated temperatures for periods of up to 15 hours. The heating and nitrogen sweeping through the gravity-flow reactor will drive the polycondensation reaction and result in longer chain lengths and, concurrently, a higher IV of the resins.
After passing through the gravity-flow reactor, pellets and/or chips of a wide range of IV can be formed, e.g., having an average IV of about 0.84 dL/g, e.g., for CSD/Beer. The pellets and/or chips have an opaque characteristic due to their crystallinity. The crystalline material is transferred to a product silo for storage and/or packaging. The finished product in a crystalline state and having an IV of about 0.84 dL/g, e.g., for CSD/Beer, can be further mixed with other co-barrier resins (powders, granules, pellets, pastilles, etc.) by molders or processors who purchase the polyester resins for manufacturing, for example, bottles and/or containers.
Thus, in a conventional process, a melt-phase polycondensation process may be used to make clear amorphous pellets (typically, 0.5-0.65 IV) as precursors to bottle resins. The amorphous pellets are first pre-crystallized, crystallized, and/or preheated, then subjected to SSP in a gravity flow reactor (e.g., a reactor that is not agitated). After crystallization, the resin pellets become opaque and do not stick together if the temperature of SSP is at least 10° C. below the onset of the melting temperature of the resin pellets. In a direct high IV melt process, only the melt process (no SSP) is used to make a variety of bottle resins (e.g., 0.72-0.78 IV for water bottles, 0.83-0.87 IV for CSD/Beer bottles) as desired. In a direct high IV melt process, a finisher (e.g., a wiped- or thin-film evaporator) may be used to effectively and rapidly remove the reaction by-products such as EG (major), water, acetaldehyde, and so on. Immediate removal of EG/water under high temperatures drives the polycondensation reaction equilibrium toward the polymer side.
PET or other polyester resins are known to have hygroscopic behavior (e.g., absorb water from the atmosphere), so pellets obtained by cutting water-quenched strands contain significant quantities of water. Conventionally, the pellets may be dried by passing dry air over the pellets or by heating. Heating for an extended period at an elevated temperature may lead to problems because the amorphous polyester (e.g., PET) pellets may have a tendency to stick to one another.
In preform molding processes, the pellets and/or chips are typically dried before molding. After proper drying, the pellets and/or chips may have a water content of not more than 50 ppm. The chips and/or pellets are then processed, for example, in the form of preforms, by injection molding. Because residual water is present in the resin during the injection molding process which is carried out at elevated temperatures (e.g., temperatures above 200° C.), the IV of the resin may be reduced, for example by hydrolytic degradation. The starting chips may be about 0.84 IV. The IV in subsequent injection-molded preforms formed from the starting resin may be about 0.80 IV. Thus, an approximate 5% reduction in IV of about 0.04 dL/g may take place in going from the chips and/or pellets to the pre-form prepared by injection molding when the chips and/or pellets have been properly dried and contain at most about 50 ppm water. Polyester material containing a greater amount of water can undergo thermal and hydrolytic degradation. Excess water in the resin can lead to a substantial reduction in IV of 30% or more.
In order to account for the loss (e.g., reduction) in IV occurring during processing, a resin having a higher IV than the IV desired for the end product must be manufactured. Typically, the difference in IV in the resin before forming a preform and the IV of the resin after forming of the preform is approximately 0.03-0.05 IV dL/g. Thus, in order to produce a molded article having a target IV of 0.80, the base resin must be first manufactured to an IV of 0.83-0.85. Because a higher IV is needed, longer polymerization times are required during the production of the base resin. Longer polymerization times result in a reduction of throughput capacity.
The particular mechanism by which the resin becomes reduced in IV during processing is not known, but it is generally understood to be attributable to one or more degradation processes including thermal, hydrolytic, oxidative, shear induced or free radical. Degradation of the resin may be accompanied by the formation of side products such as acetaldehyde.
The reduction in IV observed for some polyester resins occurs when the base resin is processed. The processing normally includes a step wherein the resin is melted and/or subjected to high shear. Such processing can include injection molding or other processing whereby the base resin is melted or transformed to a fluid state from a solid state then cooled to form a solid.
Methods of processing polyester resins which do not result in a decrease in the IV of the polyester resin would be desirable because the producer of the polyester resin may achieve greater throughput and hence productivity. Concurrently, the resin processor (e.g., injection molder) may realize greater productivity from improved processing cycle times, such as injection molding cycles, because resin with lower starting IV may require less energy for melting and may more quickly fill molds and/or be transformed into the liquid state with less shear stress relative to the shear stress that a higher IV resin may be exposed to during processing. Processing may include other types of processes with or without melting whereby the polyester resin is formed into a different shape including, for example, compression molding, stretch blow molding, thermoforming, and reaction injection molding.
Conventionally, a resin preform is transformed to a bottle or a container by blow-molding. The blow molding is carried out at a temperature above the glass transition temperature of the polyester, for example 90-110° C. which is substantially lower than the injection molding temperatures to which the pellets and/or chips are exposed during injection molding to form the pre-form. Pre-heating a preform is often carried out by infrared heating. During blow molding the IV of the resin may not change substantially, and preferably does not change at all.